The invention relates to a support comprising a supporting frame which is movably connected to a base by means of a parallelogram linkage. The linkage comprises two parallel coupling rods each of which is pivotable, around first and second parallel axes, relative to the base and the supporting frame respectively. One coupling rod comprises an arm. A constant counterbalancing force acts at a point on the arm in a direction which, extends at a first fixed angle with respect to the vertical. The direction of the force is in a plane which is transverse to the pivotal axes of the coupling rods. The supplement of this first fixed angle is substantially equal to a second fixed angle; the second fixed angle is between a first line, which connects the point at which the force acts on the arm to the first pivotal axis of the coupling rod which is provided with the arm, and a second line, which interconnects the two pivotal axes of this coupling rod. The supplement of the first fixed angle is also substantially equal to a third angle, the third angle is formed by a third line, which interconnects the second pivotal axes of the two coupling, and a fourth line, which connects the second pivotal axis of the other coupling rod to the common center of gravity of the supporting frame and a load supported thereon. The ratio of the lengths of the first and the second lines is equal to the ratio of the lengths of the third and the fourth lines.
A support of this kind is very suitable for supporting medical equipment, for example, a radiation source or a patient chair, in X-ray diagnosis and radiation-therapy apparatus.
German Pat. No. 939,348 discloses a support of the above construction in which the counterbalancing force is supplied by a counterweight which is connected to the arm. At the point of application of the counterbalancing force on the arm, the direction of the force always coincides with the vertical, regardless of the position of the supporting frame relative to the base. This counterbalancing force compensates for the weight of the supporting frame, and a load positioned thereon to the same extent regardless of the position of the supporting frame. This is because at the point of its application on the arm, the weight exerts a force whose direction coincides, like the counterbalancing force, with the vertical, regardless of the position of the supporting frame relative to the base. The force is constant because a fixed relationship exists with the weight. Therefore, for adjustment of the supporting frame a constant force is required, thus enabling accurate adjustment by hand.
A drawback of this known support is that since, counterbalancing is effected by means of a counterweight, the support is comparatively heavy when used for supporting comparatively heavy loads. For example, if the support is used for supporting a patient bed having a weight of from 250 to 500 N on which a patient having a weight of 750 N is positioned, and if the supporting frame has a weight of from 250 to 500 N and the ratio of the counterweight to the weight is from 2 to 3, then a counterweight of from 3,000 to 4,000 N must be used. When the weight of the base (from 500 to 1,000 N) is added to these weights, a total weight of from 5,000 to 7,000 N is obtained for the assembly; such a weight is unacceptable in practice.